Preparation of new materials by directed synthesis at molecular-length scales is the object of much current research in materials science, surface chemistry, and the emerging field of crystal engineering. A guiding principle of these efforts is the concept that rational synthesis using nested levels of structural hierarchy spanning a range of length scales from nanometers to the macroscopic could produce systems with tailored physical and chemical properties. The fundamental premise that underlies this approach is that the properties of a material are substantially determined by the length scales that characterize its structure and organization. Thus, the mechanical properties of nanostructured composites, the electronic properties of semiconductor clusters, the magnetic properties of metallic superlattices, and the solution properties of colloidal suspensions all correlate directly to the nanometer-scale dimensions and structure that characterize these systems. In the past ten years a number of new methods based on nanosynthetic and “crystal engineering” strategies have emerged. However, these methods remain limited both in the range of different types of building blocks that can be used, and in the extent to which molecular- and mesoscopic-scale order can be controlled. Novel approaches are needed to extend synthetic control to include new types of building blocks, for controlling molecular-scale order in thin films and crystals, and for controlling structure over macroscopic length-scales.
Liquid crystal (LC) solvents offer several advantages over conventional liquid media for solution phase synthesis of solid materials. These advantages stem from three characteristics unique to LC fluids: (1) LCs undergo strong directional coupling to solid surfaces; (2) LCs possess anisotropic (direction-dependent) properties, including various transport, optical, and mechanical properties; and (3) long-range orientational order in a LC fluid can be manipulated using an external field. Each of these characteristics can be exploited in different ways to control the structure and organization of a material prepared using LC growth media. The mechanism for controlling order in a given system depends on the type of building block and the pathway to building block aggregation.
A major challenge in the emerging fields of nanostructured materials synthesis and crystal engineering is to devise general fabrication methodologies applicable to a diverse set of fundamental building blocks and capable of producing assemblies in which structure and organization are controlled over a broad range of length scales. Interest in controlled crystallization stems from recognition that many macroscopic chemical and physical properties are determined by the microscopic arrangement of a material's basic chemical components, and by the need to prepare well ordered aggregates from building blocks that do not readily crystallize, such as some proteins. Current approaches rely either on specific intermolecular interactions to produce spontaneous self-organization, anisotropic interactions between the building block and an external field, or on a template of seed crystals or a lyotropic liquid crystal. Most methods suffer from rather severe chemical and physical constraints on the choice of fundamental building block, and the size scale characterizing structure and organization is only partly controlled.
The use of LCs as solvents for controlled crystallization and materials synthesis has not been widely studied. A method for controlling molecular alignment in an organic film through the use of a LC has been reported. See U.S. Pat. No. 5,468,519, entitled “Method For Forming an Orientation Film Including Coupling an Organic Compound to a Silane Coupling Agent in a Magnetic or Electrical Field”. This patent states that films formed by the method would be useful anchoring layers in LC-based optoelectronic devices.
Polymers are the only major class of materials at have been studied with thermotropic LCs. Some polymers, such as KEVLAR and spider silk, are thought to pass through a LC phase while curing. The resulting framework of partially oriented chains imparts various desirable properties. Partly for this reason a variety of methods have been developed to incorporate LC behavior into polymers. The most important class of these systems is liquid crystal polymers (LCP), which are synthetic polymers consisting of a flexible backbone to which small LC monomers are periodically attached. The monomers may be calamitic or discotic, and may be attached to the backbone by a linker or may be incorporated into the backbone itself. LCPs have been studied as melts, and as solutions in LC solvents. Oriented polymer materials may be formed by curing from a LC phase. Macroscopic alignment can be achieved by poling with an external alignment field. Polymer-stabilized LCs are a related system consisting of an open polymer framework filled by a LC fluid. These systems are being explored for use in LC display devices because they provide high optical contrast and they are relatively insensitive to mechanical stress and domain formation. During manufacture, polymerization to form the framework is carried out using a LC solvent in an external alignment field, resulting in partial alignment of the polymer precursors. After curing memory of the original alignment is retained by the composite. Chiral nematic solvents have also been used, although less commonly. A recent example is the polymerization of acetylene in a chiral nematic environment that resulted in helical strands whose handedness (clockwise or counterclockwise) was determined by the LC.
Several existing nanosynthetic approaches involve lyotropic liquid crystals. In one method, referred to as LC-templating (LCT), a tropic LC provides an organized scaffolding promoting condensation of an inorganic building block to form a (three-dimensional) ceramic-like framework. Inorganic precursors remain confined to the aqueous environment of the surfactant/water mixture and interact with the polar surfactant headgroups through coulombic or hydrogen-bonding forces. After condensation the organic framework may be removed, leaving a mesoporous material whose structure, pore size, and symmetry are determined by the LC scaffolding. This general approach has been creatively applied to produce several new types of nanostructured inorganic material, the most notable example perhaps being the synthesis of the M41S family of mesoporous sieves. LCT has also been used to prepare nanostructured metal clusters and patterned metallic films. These syntheses almost always result in polycrystalline materials with small grain sizes (˜μm scale).
There is growing interest in the fabrication of highly ordered molecular films for a range of applications, and considerable effort has been invested in the molecular design, synthesis, and characterization of crystalline films with targeted properties. However, a major limitation to constructing useful devices based on molecular materials, and to obtaining a better understanding of the properties of molecular solids, is that most organic compounds of interest yield polycrystalline films with random or partially random domain orientation. Numerous applications, ranging from molecular electronics and photonics to protein crystallography would benefit from a general method for growing films with uniform alignment.
Despite the advances noted above, there remains a need for highly ordered materials that can be readily formed. A need also exists for a method for readily forming a highly ordered material. The present invention seeks to fulfill these needs and provides further related advantages.